Biaxially oriented laminate polyester film

ABSTRACT

A biaxially oriented laminate polyester film (1) has a first polyester layer having a thickness (t A ) of 0.3 to 5 μm and a second polyester layer containing an inert fine particle lubricant and having a thickness (t B ) of 1.5 to 9 μm, or (2) has a first polyester layer having a thickness (t A ) of 2 to 8.5 μm and a second polyester layer containing an inert fine particle lubricant and having a thickness (t B ) of 0.6 to 5 μm. The polyester of the second polyester layer may contain a recovered polyester having the same composition as a recovered laminate polyester film which is the biaxially oriented laminate polyester film or an unstretched film thereof.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a biaxially oriented laminate polyesterfilm. More specifically, it relates to a biaxially oriented laminatepolyester film which can be self-recycled and has excellentelectromagnetic conversion characteristics, winding properties andhandling properties as a base film for a high-density magnetic recordingmedium.

A biaxially oriented polyester film typified by a polyethyleneterephthalate film is used for a wide range of application, particularlyas a base film for a magnetic recording medium due to its excellentphysical and chemical properties.

Along with recent efforts made to increase the density and capacity of amagnetic recording medium, a more flat and thinner base film is now indemand. However, when the surface of a base film is flattened to retainexcellent electromagnetic conversion characteristics, its slipperinessbecomes insufficient. For instance, when the base film is rolled up, itis wrinkled or blocking occurs. As a result, the surface of the filmroll becomes uneven, whereby the yield of products is reduced, orappropriate ranges of tension, contact pressure and speed for winding upthe base film are narrowed, thereby making it extremely difficult towind up the base film. When the slipperiness of the base film is low inthe film processing step, the friction of the base film with a metalroll in contact with the base film increases, thereby generatingchippings which cause lack of a magnetic recording signal, that is, adrop-out.

To improve the slipperiness of a polyester film, the following methodsare generally employed: (i) one in which inert particles are depositedinto a raw material polymer from the residual catalyst in the productionprocess and (ii) one in which the surface of a film is made uneven byadding inert particles to the film. Generally speaking, the greater thesize or content of particles in the film the more the slipperiness isimproved.

As described above, the surface of a base film is desired to be as evenas possible in order to improve electromagnetic conversioncharacteristics. When a magnetic recording medium is formed from a basefilm having a rough surface, the roughness of the surface of the basefilm appears on the surface of a magnetic layer even after the formationof the magnetic layer, thereby deteriorating electromagnetic conversioncharacteristics. In this case, the larger the size or content ofparticles contained in the base film the greater the surface roughnessof the base film becomes and the more the electromagnetic conversioncharacteristics deteriorate.

To improve both winding properties and electromagnetic conversioncharacteristics which are conflicting properties, there is widely knownmeans of producing a laminate film having an even surface on which amagnetic layer is formed for improving electromagnetic conversioncharacteristics and an opposite rough surface for improvingslipperiness.

For the flat layer on which the magnetic layer is formed, a lubricanthaving a small particle diameter is used or the amount of the lubricantadded is reduced to flatten the layer, whereas for the rough layer onthe opposite side (running side) on which no magnetic layer is formed, alubricant having a large particle diameter is used or the amount of thelubricant added is increased to roughen the layer.

That is, the flat layer on the coated side and the rough layer on therunning side greatly differ from each other in the characteristicproperties of a lubricant used, e.g., the type, particle diameter andamount of the lubricant.

In the case of a single-layer film, film waste generated in theproduction process of a film is recovered and formed into a chip whichcan be recycled for the production of the film. In the case of the abovelaminate film, the lubricant composition of a chip recovered from thelaminate film differs from the lubricant compositions of a rough layerand a flat layer. Therefore, when the recovered chip is recycled for theproduction of a laminate film, the lubricant composition of a layer madefrom the recovered chip changes and affects the characteristicproperties of the film.

It has recently been proposed to reuse a chip recovered from thelaminate film for an intermediate layer portion (core layer portion) ofa three-layer laminate film.

However, in this method, the intermediate layer portion must be thickenough to enable the recovery of a chip from the laminate film inaddition to the recovery of a chip from the three-layer laminate film.Therefore, the three-layer laminate film must be made extremely thick.Even when a recovered chip containing a lubricant having a largeparticle diameter or a large amount of a lubricant is used in anintermediate portion, it influences the formation of protrusions on asurface layer portion. Therefore, the use of the chip is limited.

As described above, a magnetic recording medium having a higher densityand a larger capacity and a base film having a smaller thickness haverecently been desired. Therefore, the above three-layer laminate filmbecomes also thin and accordingly, it is substantially difficult toreuse a polymer (chip) recovered from the above laminate film in theintermediate layer of the above three-layer laminate film.

Therefore, as matters now stand, the polymer recovered from the laminatefilm is inevitably discarded, thereby boosting the costs of the film.Such discarded films are disposed of as industrial waste but it isbecoming difficult to dispose of such films at present.

It is therefore an object of the present invention to provide abiaxially oriented laminate polyester film which can be self-recycled,has excellent winding and handling properties and is useful as a basefilm for a high-density magnetic recording medium having excellentelectromagnetic conversion characteristics.

The other objects and advantages of the present invention will becomeapparent from the following description.

According to the present invention, firstly, the above objects andadvantages of the present invention can be attained by a biaxiallyoriented laminate polyester film (may be referred to as “first laminatefilm of the present invention” hereinafter) comprising a first polyesterlayer and a second polyester layer, wherein the first polyester layerhas a thickness (t_(A)) of 0.3 to 5 μm, the second polyester layercontains an inert fine particle lubricant and has a thickness (t_(B)) of1.5 to 9 μm, and the first polyester layer and the second polyesterlayer satisfy the following expressions (1) to (4):

WRa(B)>WRa(A)  (1)

0.5≦t _(B) /t≦0.9  (2)

10<t _(B) /d _(B)≦60  (3)

t=3 to 10 μm  (4)

wherein WRa(A) is the center plane average roughness (nm) of the exposedsurface of the first polyester layer, WRa(B) is the center plane averageroughness (nm) of the exposed surface of the second polyester layer,t_(B) is the thickness (μm) of the second polyester layer, t is the sumof t_(A) and t_(B), t_(A) is the thickness (μm) of the first polyesterlayer, and d_(B) is the average particle diameter (μm) of the inert fineparticle lubricant contained in the second polyester layer.

Secondly, the above objects and advantages of the present invention areattained by a biaxially oriented laminate polyester film (may bereferred to as “second laminate film of the present invention”hereinafter) comprising a first polyester layer and a second polyesterlayer, wherein

the first polyester layer has a thickness (t_(A)) of 2 to 8.5 μm, thesecond polyester layer contains an inert fine particle lubricant and hasa thickness (t_(B)) of 0.6 to 5 μm, and the first polyester layer andthe second polyester layer satisfy the following expressions (1) to(4′):

WRa(B)>WRa(A)  (1)

0.15≦t _(B) /t<0.5  (2′)

10<t _(B) /d _(B)≦45  (3′)

t=4 to 10 μm  (4′)

wherein WRa(A) is the center plane average roughness (nm) of the exposedsurface of the first polyester layer, WRa(B) is the center plane averageroughness (nm) of the exposed surface of the second polyester layer,t_(B) is the thickness (μm) of the second polyester layer, t is the sumof t_(A) and t_(B), t_(A) is the thickness (μm) of the first polyesterlayer, and d_(B) is the average particle diameter (μm) of the inert fineparticle lubricant contained in the second polyester layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram for explaining a self-recycling method; and

FIG. 2 is a typified diagram showing an apparatus for measuring thevolume resistivity of a molten polymer.

The characteristic features of the first laminate film of the presentinvention will be roughly described hereinafter. The first laminate filmis a laminate film which consists of a rough layer (second polyesterlayer) and a flat layer (first polyester layer) thinner than the roughlayer, rarely experiences changes in its surface properties even when arecovered film by-produced at the time of manufacturing the laminatefilm is self-recycled and used for the production of the laminate filmby setting the ratio of the thickness of the rough layer to the averageparticle diameter of a lubricant contained in the rough layer to aspecific range, and moreover, has excellent electromagnetic conversioncharacteristics and winding properties as a base film for a high-densitymagnetic recording medium.

The characteristic features of the second laminate film of the presentinvention will be roughly described hereinafter. The second laminatefilm is a laminate film which consists of a rough layer (secondpolyester layer) and a flat layer (first polyester layer) thicker thanthe rough layer, rarely experiences changes in its surface propertieseven when a recovered film by-produced at the time of manufacturing thelaminate film is self-recycled and used for the production of thelaminate film by setting the ratio of the thickness of the rough layerto the average particle diameter of a lubricant contained in the roughlayer to a specific range, and moreover, has excellent electromagneticconversion characteristics and winding properties as a base film for ahigh-density magnetic recording medium.

A description is first given of the first laminate film of the presentinvention.

The polyester used in the first and second polyester layers of thepresent invention is preferably an aromatic polyester comprising anaromatic dicarboxylic acid as the main acid component and an aliphaticglycol as the main glycol component. The aromatic polyester issubstantially linear and has film formability, especially filmformability by melt molding.

Illustrative examples of the aromatic dicarboxylic acid includeterephthalic acid, 2,6- and 2,7-naphthalenedicarboxylic acids,isophthalic acid, diphenoxyethane dicarboxylic acid,diphenyldicarboxylic acid, diphenyl ether dicarboxylic acid, diphenylsulfone dicarboxylic acid, diphenyl ketone dicarboxylic acid,anthracenedicarboxylic acid and the like. Illustrative examples of thealiphatic glycol include polymethylene glycols having 2 to 10 carbonatoms such as ethylene glycol, trimethylene glycol, tetramethyleneglycol, pentamethylene glycol, hexamethylene glycol and decamethyleneglycol, and alicyclic diols such as cyclohexane dimethanol.

The aromatic polyester preferably comprises an alkylene terephthalate oran alkylene naphthalate as the main constituent. The polyester isparticularly preferably polyethylene terephthalate,polyethylene-2,6-naphthalate or a copolymer which comprises terephthalicacid or 2,6-naphthalenedicarboxylic acid in an amount of 80 mol % ormore of the total of all the dicarboxylic acid components and ethyleneglycol in an amount of 80 mol % or more of the total of all the glycolcomponents. In the latter copolymer, 20 mol % or less of the total ofall the acid components may be the above aromatic dicarboxylic acidother than terephthalic acid or 2,6-naphthalenedicarboxylic acid,aliphatic dicarboxylic acid such as adipic acid or sebacic acid, oralicyclic dicarboxylic acid such as cyclohexane-1,4-dicarboxylic acid.20 mol % or less of the total of all the glycol components may be theabove glycol other than ethylene glycol, aromatic diol such ashydroquinone, resorcin or 2,2-bis(4-hydroxyphenyl)propane, aliphaticdiol having an aromatic ring such as 1,4-dihydroxydimethylbenzene, orpolyalkylene glycol (polyoxyalkylene glycol) such as polyethyleneglycol, polypropylene glycol or polytetramethylene glycol.

The aromatic polyester of the present invention includes polyestersobtained by copolymerizing or bonding a component derived from anoxycarboxylic acid such as an aromatic oxyacid exemplified byhydroxybenzoic acid or an aliphatic oxyacid exemplified by(ω-hydroxycaproic acid in an amount of 20 mol % or less based on thetotal of all the dicarboxylic acid components and all the oxycarboxylicacid components.

The aromatic polyester of the present invention further includescopolymers comprising a polycarboxylic acid or polyhydroxy compoundhaving 3 or more functional groups, such as trimellitic acid orpentaerythritol, in such an amount that it is substantially linear, forexample, in an amount of 2 mol % or less of the total of all the acidcomponents.

The aromatic polyester of the present invention preferably comprises, asthe copolymerized component, a sulfonic acid quaternary phosphonium saltin an amount of 0.02 to 45 mmol % (based on the total of all the acidcomponents of the polyester). The copolymerization of this sulfonic acidquaternary phosphonium salt in the above amount makes it possible toaccelerate the casting speed and retain the discharge function of adischarge electrode in the electrostatic contacting of a film. With theabove amount of the sulfonic acid quaternary phosphonium salt, the ACvolume resistivity of a molten film can be adjusted to 1×10⁶ to 9×10⁸Ω·cm. When this AC volume resistivity is higher than 9×10⁸ Ω·cm, theeffect of accelerating the casting speed is small, while when the ACvolume resistivity is lower than 1×10⁶ Ω·cm, the breakdown of a filmoccurs at the time of electrostatic contacting in the casting step,undesirably.

The sulfonic acid quaternary phosphonium salt to be copolymerized ispreferably a compound represented by the following formula:

wherein A is an aromatic or aliphatic tervalent residual group, X₁ andX₂ are the same or different ester-forming functional groups or hydrogenatom, R₁, R₂, R₃ and R₄ are the same or different groups selected fromthe group consisting of an alkyl group and an aryl group, and n is apositive integer.

Preferred examples of the above sulfonic acid phosphonium salt includetetrabutylphosphonium 3,5-dicarboxybenzene sulfonate,tetraphenylphosphonium 3,5-dicarboxybenzene sulfonate,tetrabutylphosphonium 3,5-dicarboxymethoxybenzene sulfonate,tetraphenylphosphonium 3,5-dicarboxymethoxybenzene sulfonate,tetrabutylphosphonium 3-carboxybenzene sulfonate, tetrabutylphosphonium3,5-di(β-hydroxyethoxycarbonyl)benzene sulfonate, tetrabutylphosphonium4-hydroxyethoxybenzene sulfonate, bisphenolA-3,3′-di(tetrabutylphosphonium sulfonate), tetrabutylphosphonium2,6-dicarboxynaphthalene-4-sulfonate and the like. They may be usedalone or in combination of two or more. The method of copolymerizing theabove sulfonic acid quaternary phosphonium salt is not particularlylimited. For example, a polyester is polymerized in the presence of asulfonic acid quaternary phosphonium salt compound to contain thecompound in the polymer chain of a polyester, or a sulfonic acidquaternary phosphonium salt compound is injected and melt kneaded in anextruder simultaneous with a polyester when the polyester is injectedinto the extruder to be molten and extruded.

The above aromatic polyester is known per se and can be produced by aknown method per se.

The above aromatic polyester preferably has an intrinsic viscosity,measured at 35° C. in o-chlorophenol, of 0.4 to 0.9, more preferably 0.5to 0.7, particularly preferably 0.51 to 0.65.

The biaxially oriented laminate polyester film of the present inventionconsists of the first polyester layer and the second polyester layer.The polyesters of the two layers may be the same or different,preferably the same.

The biaxially oriented laminate polyester film of the present inventionis a laminate polyester film which can be self-recycled and has adifference in surface roughness between the both surfaces. The recoveredlaminate polyester film can be used as part of a polymer forming thesecond polyester layer.

The biaxially oriented laminate polyester film of the present inventioncomprising the first polyester layer and the second polyester layerformed on the first polyester layer and must satisfy the followingexpressions (1) to (4):

WRa(B)>WRa(A)  (1)

0.5≦t _(B) /t≦0.9  (2)

10<t _(B) /d _(B)≦60  (3)

t=3 to 10 μm  (4)

wherein WRa(A) is the center plane average roughness (nm) of the exposedsurface of the first polyester layer, WRa(B) is the center plane averageroughness (nm) of the exposed surface of the second polyester layer,t_(B) is the thickness (μm) of the second polyester layer, t is the sumof t_(A) and t_(B), t_(A) is the thickness (μm) of the first polyesterlayer, and d_(B) is the average particle diameter (μm) of inert fineparticles contained in the second polyester layer.

More preferably, the polyester of the second polyester layer is arecovered polyester having the same composition as a recovered laminatepolyester film and the recovered laminate polyester film is thebiaxially oriented laminate polyester film of the present invention orunstretched film thereof.

When the above recovered polyester is used in the first polyester layer,namely, the flat layer, in the production of a biaxially orientedlaminate polyester film, a large inert particle lubricant contained inthe rough layer (second polyester layer) for providing windingproperties is contained in the first polyester layer, whereby highprotrusions are formed on the flat layer (first polyester layer) tocause deterioration electromagnetic conversion characteristics, andhence, the obtained biaxially oriented laminate polyester film is notsuitable as a base film for a high-density magnetic recording medium.

In the present invention, the value of (t_(B)/t) must be in the range of0.5 to 0.9. When the value is smaller than 0.5, the proportion of apolymer recovered from the laminate polyester film and usable for theformation of the rough layer lowers because the concentration of theinert fine particle lubricant contained in the recovered polymer islower than the concentration of the inert fine particle lubricantcontained in the second polyester layer due to the existence of the flatlayer and accordingly, the concentration of the inert fine particlelubricant must be compensated to an appropriate level by supplementing anew polymer having a high concentration of the inert fine particlelubricant. As a result, the proportion of the recovered polymer issmaller than 50%. Further, the production cost of the biaxially orientedlaminate polyester film increases and the application range of the filmto the market narrows.

On the other hand, when the value of (t_(B)/t) is larger than 0.9, theflat layer becomes thin, whereby the inert fine particle lubricantcontained in the rough layer exerts an influence on the flat layer toroughen its flat surface. As a result, electromagnetic conversioncharacteristics deteriorate and hence, the obtained biaxially orientedlaminate polyester film is not suitable as a base film for ahigh-density magnetic recording medium.

In the present invention, t_(B)/d_(B) must be in the range of 10 to 60.When t_(B)/d_(B) is smaller than 10, that is, the thickness of the roughlayer is too small, or when the average particle diameter of the inertfine particle lubricant contained in the rough layer is too large, theamount of the recovered polymer which can be recycled for the formationof the rough layer decreases in the former case, with the consequencethat the production cost of the film increases and the application rangeof the film in the market narrows, whereas in the latter case, the inertfine particle lubricant contained in the rough layer exerts an influenceon the flat layer to roughen its flat surface, with the consequence thatelectromagnetic conversion characteristics thereby deteriorate and theobtained biaxially oriented laminate polyester film is not suitable as abase film for a high-density magnetic recording medium.

When t_(B)/d_(B) is large than 60, that is, the average particlediameter of the inert fine particle lubricant contained in the roughlayer is too small for the thickness of the rough layer, protrusionsformed on the rough layer are too low, thereby making it impossible toobtain satisfactory winding properties.

In the present invention, the second polyester layer preferably containstwo or more kinds of inert fine particle lubricants having differentaverage particle diameters. Preferably, two or more kinds of inert fineparticle lubricants having different average particle diameters are oftwo or more different chemical species or the same chemical species andhave clearly distinguishable particle size distributions. Particularlypreferably, the second polyester layer contains three kinds of inertfine particles having different average particle diameters as amulti-component lubricant. The above particles include a small amount ofmedium-sized particles having a medium average particle diameter and alarger amount of small-sized particles than the medium-sized particlesto provide slipperiness. When only small-sized particles are used as asingle-component lubricant, sufficient air-squeeze properties cannot beobtained and winding properties and handling properties becomeunsatisfactory. When only medium- to large-sized particles are used inlarge quantities as a single-component lubricant, electromagneticconversion characteristics deteriorate and when they are used in smallquantities, film slipperiness becomes unsatisfactory. Therefore, both ofthe above characteristic properties can be hardly obtainedsimultaneously with a single-component lubricant.

The inert fine particle lubricant is preferably heat resistant polymerparticles and/or spherical silica particles. More preferably, the inertfine particle lubricant contains heat resistant polymer particles asmedium-sized particles and spherical silica particles as small-sizedparticles. Illustrative examples of the heat resistant polymer particlesinclude crosslinked polystyrene resin particles, crosslinked siliconeresin particles, crosslinked acrylic resin particles, crosslinkedstyrene-acrylic resin particles, crosslinked polyester particles,polyimide particles, melamine resin particles and the like. Whencrosslinked polystyrene resin particles or crosslinked silicone resinparticles is contained out of these, the effect of the present inventionbecomes more marked advantageously.

When the above heat resistant polymer particles and spherical silicaparticles are used, protrusions having affinity for polyesters andrelatively uniform in size are formed, thereby improving theslipperiness, chipping resistance and electromagnetic conversioncharacteristics of the film.

Preferably, the biaxially oriented laminate polyester film of thepresent invention contains a lubricant in at least the rough layer(second polyester layer) as described above, and the rough layercontains part of the polymer (recovered polymer) of a film which isby-produced and recovered at the time of producing the biaxiallyoriented laminate polyester film. When the second polyester layer isformed from the recovered polymer and a new polymer, the concentration(C_(Bi), wt %) of the inert fine particle lubricant contained in thesecond polyester layer desirably satisfies the following equation:

C _(Bi)=(C _(Ai) ×t _(A) ×R+100×C _(vi)×(t _(B)−(t _(A) +t_(B))×R/100))/(t _(B)×(100−R)

wherein C_(Ai) is the concentration (wt %) of an inert fine particlelubricant contained in the first polyester layer of the recoveredlaminate polyester film, C_(vi) is the concentration (wt %) of an inertfine particle lubricant contained in the new polyester used togetherwith the recovered laminate polyester film for the formation of thesecond polyester layer, t_(A) is the thickness (μm) of the firstpolyester layer of the recovered laminate polyester film, t_(B) is thethickness (μm) of the second polyester layer of the recovered laminatepolyester film, and R is the proportion (wt %) of the recovered laminatepolyester film used together with the new polyester for the formation ofthe second polyester layer.

R is preferably 1 wt % or more to 90 wt % or less. The lowest value ismore preferably 5 wt %, particularly preferably 10 wt % and extremelypreferably 30 wt %. Further, the highest value is more preferably 85 wt%, particularly preferably 80 wt % and extremely preferably 70 wt %.

The above equation will be described with reference to FIG. 1.

When the biaxially oriented laminate polyester film of the presentinvention is manufactured and provided as a product, a biaxiallyoriented laminate polyester film or unstretched film thereof which hasthe same constitution or composition as the above biaxially orientedlaminate polyester film is by-produced and recycled as a recovered chip.This recovered chip contains a lubricant “i” in a concentration lowerthan the concentration (C_(Bi)) of the lubricant “i” contained in thesecond polyester layer due to the existence of the flat layer (firstpolyester layer) containing no lubricant “i”. Therefore, when thisrecovered chip is used, a new polyester containing the lubricant “i” ina concentration C_(vi) higher than C_(Bi) is used in combination toadjust the concentration of the lubricant “i” contained in the secondpolyester layer to C_(Bi). That is, this adjustment is carried out bythe above equation.

A recovered laminate polyester film which has the same composition ofthe lubricant “i” and the same thickness ratio of the first polyesterlayer to the second polyester layer as a product polyester film but hasa total thickness different from that of the product polyester film maybe also used as the recovered polymer (recovered chip) in FIG. 1, thougha recovered laminate polyester film having the same laminate structureand composition as the laminate polyester film of the present inventionis preferably used. The intrinsic viscosity of the polymer of the secondpolyester layer is preferably smaller than that of the polymer of thefirst polyester layer from the viewpoint of production cost. Accordingto circumstances, the intrinsic viscosities of the polymers of the firstand second polyester layers may be made the same value by setting theintrinsic viscosity of the new polymer used in the second polyesterlayer slightly higher than the intrinsic viscosity of the firstpolyester layer and controlling the proportion and intrinsic viscosityof the recovered polymer.

Preferably, the biaxially oriented laminate polyester film contains oneor more different kinds of lubricants in the first polyester layer andtwo or more different kinds of lubricants in the second polyester layer.However, the present invention is not limited to this. Any biaxiallyoriented laminate polyester film is acceptable if it contains nolubricant in the first polyester layer and one kind of lubricant in thesecond polyester layer and satisfies the above expressions.

The aromatic polyester in the present invention is preferablypolyethylene terephthalate or polyethylene-2,6-naphthalate. To obtain abiaxially oriented laminate film having a thickness of 3 to 6 μm andhigh Young's moduli, polyethylene-2,6-naphthalate is more preferred.

The surface roughness of the rough layer (second polyester layer) andthe surface roughness of the flat layer (first polyester layer) of thebiaxially oriented laminate polyester film of the present invention arenot particularly limited. However, when the biaxially oriented laminatepolyester film is used as a base film for a high-density magneticrecording medium, particularly a high-density digital recording medium,the surface roughness (WRa(A)) of the first polyester layer ispreferably 3 to 8 nm, more preferably 4 to 8 nm, particularly preferably5 to 7 nm. When WRa(A) is more than 8 nm, satisfactory electromagneticconversion characteristics are hardly obtained. WhenWRa(A) is less than3 nm, film slipperiness deteriorates and a sufficiently high slit yieldis hardly obtained, and the slipperiness of a flat surface with a passroll system in the production process of a film or tape worsens and thefilm or tape is wrinkled due to defective conveyance, thereby greatlyreducing the product yield.

WRa(B) of the second polyester layer which is the rough layer ispreferably 6 to 18 nm, more preferably 7 to 17 nm, particularlypreferably 9 to 15 nm. When WRa(B) is less than 6 nm, film slipperinessdeteriorates and a sufficiently high slit yield is hardly obtained. WhenWRa(B) is more than 18 nm, the influence of protrusions on the flatsurface becomes large with the result of the roughened flat surface, andsatisfactory electromagnetic conversion characteristics are hardlyobtained.

Preferably, the biaxially oriented laminate polyester film of thepresent invention has Young's moduli in longitudinal and transversedirections each of 450 to 2,000 kg/mm² and the ratio of the Young'smodulus in a longitudinal direction to the Young's modulus in atransverse direction is 0.3 to 2.5. The each of Young's moduli inlongitudinal and transverse directions are more preferably 500 to 1,200kg/mm², particularly preferably 600 to 900 kg/mm². The ratio of theYoung's moduli is more preferably 0.4 to 2.0, particularly preferably0.6 to 1.6.

When the Young's modulus in a longitudinal direction of the film is lessthan 450 kg/mm², the strength in a longitudinal direction of a magnetictape becomes low, whereby the tape is easily broken when it is set in amagnetic recording device and great force is applied to it in alongitudinal direction. When the Young's modulus in a transversedirection is less than 450 kg/mm², the strength in a transversedirection of a magnetic tape becomes low, whereby contact between thetape and the magnetic head becomes weak, thereby making it difficult toobtain satisfactory electromagnetic conversion characteristics.Meanwhile, when the Young's modulus in a longitudinal or transversedirection is more than 2,000 kg/mm², the resulting film easily breaksvery often due to a high draw ratio at the time of film formation,thereby reducing the product yield.

When the ratio of the Young's modulus in a longitudinal direction to theYoung's modulus in a transverse direction is less than 0.3, theresulting magnetic tape hardly obtains sufficient strength in alongitudinal direction. As a result, when it is set in a magneticrecording device and great force is exerted to it in a longitudinaldirection, it is easily break frequently. When the ratio is more than2.5, the resulting magnetic tape hardly obtains sufficient strength in atransverse direction. As a result, when the tape is caused to run, tapeedges are easily damaged and satisfactory durability is hardly obtained.

The ratio of the Young's modulus in a longitudinal direction to that ina transverse direction is preferably 0.9 to 2.5 when the biaxiallyoriented laminate polyester film is used as a base film for a magneticrecording medium of a linear system and 0.3 to 1.0 when the film is usedas a base film for a magnetic recording medium of a helical system.

The total thickness of the biaxially oriented laminate polyester film ofthe present invention is 3 to 10 μm. This thickness is advantageous whenit is used as a base film for a high-density magnetic recording medium.The thickness is preferably 4 to 9 μm, particularly preferably 4 to 7μm. When the thickness is larger than 10 μm, the length of a magnetictape which can be stored in a cassette becomes short and accordingly, asufficient recording capacity cannot be obtained. When the thickness issmaller than 3 μm, the film often breaks at the time of film formationand film winding properties deteriorate, resulting in a great reductionin yield.

The biaxially oriented laminate polyester film of the present inventionmay be manufactured in accordance with a conventionally known method ora method which has been accumulated by the industry. For example, it canbe obtained by first forming an unstretched laminate film and then,biaxially stretching the film. This unstretched laminate film can beproduced by a laminate film production method which has been accumulatedheretofore. For example, the second polyester layer forming a roughsurface and the first polyester layer forming an opposite surface (flatsurface) are laminated together while the polyesters are molten orsolidified by cooling. Stated more specifically, it can be produced bycoextrusion, extrusion lamination or the like.

According to the present invention, there is provided the followingmethod as a method of producing the biaxially oriented polyester film ofthe present invention, wherein a recovered laminate polyester film isused as one of raw materials for the formation of the second polyesterlayer of the biaxially oriented laminate polyester film of the presentinvention.

The method of producing a biaxially oriented laminate polyester film bybiaxially stretching an unstretched laminate polyester film consistingof a first unstretched polyester layer and a second unstretchedpolyester layer, wherein

a recovered laminate polyester film and a new polyester are used to formthe second unstretched polyester layer under the condition that thefollowing expression should be satisfied:

C _(Bi)=(C _(Ai) ×t _(A) ×R+100×C _(vi)(t _(B)−(t _(A) +t_(B))×R/100))/(t _(B)×(100−R))

wherein C_(Bi) is the concentration (wt %) of an inert fine particlelubricant contained in the second unstretched polyester layer, C_(Ai) isthe concentration (wt %) of an inert fine particle lubricant containedin the first polyester layer of the recovered laminate polyester film,C_(vi) is the concentration (wt %) of an inert fine particle lubricantcontained in the new polyester used together with the recovered laminatepolyester film for the formation of the second polyester layer, t_(A) isthe thickness (μm) of the first polyester layer of the recoveredlaminate polyester film, t_(B) is the thickness (μm) of the secondpolyester layer of the recovered laminate polyester film, and R is theproportion (wt %) of the recovered laminate polyester film used togetherwith the new polyester for the formation of the second polyester layer.

The unstretched laminate film obtained by the above method can be formedinto a biaxially oriented laminate polyester film by a biaxiallyoriented film production method which has been accumulated heretofore.For example, polyesters are molten and coextruded at a temperature of amelting point (Tm: ° C.) to (Tm+70)° C. to obtain an unstretchedlaminate film which is then stretched uniaxially (in a longitudinaldirection or transverse direction) to 2.5 times or more, preferably 3times or more, at a temperature of (Tg−10) to (Tg+70)° C. (Tg: glasstransition temperature of polyesters) and then in a directionperpendicular to the above stretching direction to 2.5 times or more,preferably 3.0 times or more, at a temperature of Tg to (Tg+70)° C. Itmay be further stretched in a longitudinal direction and/or transversedirection again as required. The total stretch ratio is preferably 9times or more, more preferably 12 to 35 times, particularly preferably15 to 30 times in terms of area stretch ratio. Further, the biaxiallyoriented laminate film may be heat set at a temperature of (Tg+70) to(Tm−10)° C., preferably 180 to 250° C. The heat setting time ispreferably 1 to 60 seconds.

Preferably, the biaxially oriented laminate polyester film of thepresent invention contains a sulfonic acid quaternary phosphonium saltin an amount of 0.02 to 45 mmol % and has an AC volume resistivity of1×10⁶ to 9×10⁸ Ω·cm, as described above. That is, pinning properties areimproved at the time of film formation and high-speed film formation ismade possible by containing the sulfonic acid quaternary phosphoniumsalt in the above amount.

The biaxially oriented laminate polyester film of the present inventionis preferably used as a base film for a high-density magnetic recordingmedium or a high-density digital recording medium (data cartridge,digital video tape or the like). More specifically, it is advantageouslyused as a base film for a magnetic recording tape for digital recordingmode or a magnetic recording tape for data storage.

A description is subsequently given of the second laminate film of thepresent invention. As for what is not described hereinafter, it shouldbe understood that what has been described of the first laminate film ofthe present invention is applied directly or with slight modificationobvious to one having ordinary skill in the art.

The biaxially oriented laminate polyester film of the present inventioncomprising a first polyester layer and a second polyester layer formedon the first polyester layer and must satisfy the following expressions(1) to (4′):

WRa(B)>WRa(A)  (1)

0.15≦t _(B) /t<0.5  (2′)

10<t _(B) /d _(B)≦45  (3′)

t=4 to 10 μm  (4′)

wherein WRa(A) is the center plane average roughness (nm) of the exposedsurface of the first polyester layer, WRa(B) is the center plane averageroughness (nm) of the exposed surface of the second polyester layer,t_(B) is the thickness (μm) of the second polyester layer, t is the sumof t_(A) and t_(B), t_(A) is the thickness (μm) of the first polyesterlayer, and d_(B) is the average particle diameter (μm) of the inert fineparticle lubricant contained in the second polyester layer.

In the present invention, when the value of t_(B)/t is smaller than0.15, the proportion of a polymer recovered from the laminate polyesterfilm and usable for the formation of a second polyester layer lowersbecause the concentration of the inert fine particle lubricant containedin the recovered polymer is lower than the concentration of the inertfine particle lubricant contained in the second polyester layer due tothe existence of the flat layer and accordingly, the concentration ofthe inert fine particle lubricant must be compensated to an appropriatelevel by supplying a new polymer having a high concentration of theinert fine particle lubricant. As a result, the proportion of therecovered polymer is smaller than 15 %. Further, the production cost ofthe biaxially oriented laminate polyester film increases and theapplication range of the film to the market narrows.

When the value of (t_(B)/t) is larger than 0.5, the flat layer becomesthin, whereby the inert fine particle lubricant contained in the roughlayer exerts an influence on the flat layer to roughen its flat surface.As a result, the magnetic surface is roughened to worsen electromagneticconversion characteristics. Particularly in a high-density magneticrecording medium having a very thin magnetic layer, this roughened flatsurface worsen electromagnetic conversion characteristics and hence, theobtained biaxially oriented laminate polyester film is not suitable as abase film for the high-density magnetic recording medium.

In the present invention, t_(B)/d_(B) must be in the range of 10 to 45.When t_(B)/d_(B) is smaller than 10, that is, the thickness of the roughlayer is too small, or when the average particle diameter of the inertfine particle lubricant contained in the rough layer is too large, theamount of the recovered polymer which can be recycled for the formationof the rough layer decreases in the former case, with the consequencethat the production cost of the film increases and the application rangeof the film in the market narrows, whereas in the latter case, the inertfine particle lubricant contained in the rough layer exerts an influenceon the flat layer to roughen its flat surface, with the consequence thatelectromagnetic conversion characteristics deteriorate and the obtainedbiaxially oriented laminate polyester film is not suitable as a basefilm for a high-density magnetic recording medium.

When t_(B)/d_(B) is large than 45, that is, the average particlediameter of the inert fine particle lubricant contained in the roughlayer is too small for the thickness of the rough layer, protrusionsformed on the rough layer are too low, thereby making it impossible toobtain satisfactory winding properties.

The biaxially oriented laminate polyester film of the present inventioncontains a lubricant in at least the rough layer (second polyesterlayer) as described above and the rough layer contains part of thepolymer (recovered polymer) of a film which is by-produced and recoveredat the time of producing the biaxially oriented laminate polyester film.When the second polyester layer is formed from the recovered polymer anda new polymer, the concentration (C_(Bi), wt %) of the inert fineparticle lubricant contained in the second polyester layer desirablysatisfies the following equation:

C _(Bi)=(C _(Ai) ×t _(A) ×R+100×C _(vi)×(t _(B)−(t _(A) +t_(B))×R/100))/(t _(B)×(100−R))

wherein C_(Ai), t_(A), R, C_(vi) and t_(B) are as defined hereinabove.

R is preferably 1 wt % or more to 50 wt % or less. The lowest value ismore preferably 5 wt %, particularly preferably 10 wt % and extremelypreferably 20 wt %. Further, the highest value is more preferably 45 wt%, particularly preferably 40 wt % and extremely preferably 30 wt %.

The surface roughness of the rough layer (second polyester layer) andthe surface roughness of the flat layer (first polyester layer) of thebiaxially oriented laminate polyester film of the present invention arenot particularly limited. When the biaxially oriented laminate polyesterfilm is used as a base film for a high-density magnetic recordingmedium, particularly a high-density digital recording medium, however,the surface roughness (WRa(A)) of the first polyester layer ispreferably 1 to 5 nm, more preferably 1 to 4 nm, particularly preferably2 to 4 nm. When WRa(A) is more than 5 nm, satisfactory electromagneticconversion characteristics are hardly obtained. When WRa(A) is less than1 nm, film slipperiness deteriorates and a sufficiently high slit yieldis hardly obtained, and the slipperiness of a flat surface with a passroll system in the production process of a film or tape worsens and thefilm or tape is wrinkled due to poor conveyance, thereby greatlyreducing the product yield.

WRa(B) of the second polyester layer which is the rough layer ispreferably 5 to 20 nm, more preferably 7 to 17 nm, particularlypreferably 9 to 15 nm. When WRa(B) is less than 5 nm, film slipperinessdeteriorates and a sufficiently high slit yield is hardly obtained. WhenWRa(B) is more than 20 nm, the influence of protrusions on the flatsurface becomes large with the result of the roughened flat surface, andsatisfactory electromagnetic conversion characteristics are hardlyobtained.

According to the present invention, there is provided the followingmethod as a method of producing the biaxially oriented polyester film ofthe present invention, wherein a recovered laminate polyester film isused as one of raw materials for the formation of the second polyesterlayer of the biaxially oriented laminate polyester film of the presentinvention.

The method of producing a biaxially oriented laminate polyester film bybiaxially stretching an unstretched laminate polyester film consistingof a first unstretched polyester layer and a second unstretchedpolyester layer, wherein

a recovered laminate polyester film and a new polyester are used to formthe second unstretched polyester layer under the condition that thefollowing expression should be satisfied:

 C _(Bi)=(C _(Ai) ×t _(A) ×R+100×C _(vi)(t _(B)−(t _(A) +t_(B))×R/100))/(t _(B)×(100−R))

wherein C_(Bi) is the concentration (wt %) of an inert fine particlelubricant contained in the second unstretched polyester layer, C_(Ai) isthe concentration (wt %) of an inert fine particle lubricant containedin the first polyester layer of the recovered laminate polyester film,C_(vi) is the concentration (wt %) of an inert fine particle lubricantcontained in the new polyester used together with the recovered laminatepolyester film for the formation of the second polyester layer, t_(A) isthe thickness (μm) of the first polyester layer of the recoveredlaminate polyester film, t_(B) is the thickness (μm) of the secondpolyester layer of the recovered laminate polyester film, and R is theproportion (wt %) of the recovered laminate polyester film used togetherwith the new polyester for the formation of the second polyester layer.

EXAMPLE

The following examples are given to further illustrate the presentinvention.

Various physical property values and characteristic properties in thepresent invention were measured and defined as follows.

(1) Average Particle Diameter of Particles (DP)

The polyester is removed from the surface of the film by alow-temperature plasma ashing method (for example, model P3-3 of YamatoKagaku Co., Ltd.) to expose particles. The treatment conditions areselected to ensure that the polyester is ashed and the particles are notdamaged. The exposed particles are observed through a SEM (ScanningElectron Microscope) to analyze an image (light and shade formed by theparticles) of the particles with an image analyzer. The followingnumerical processing is carried out with 5,000 or more particles bychanging the observation site and the number average particle diameter dobtained by the equation (5) is taken as the average particle diameter.

d=Σdi/n  (5)

wherein di is the circle equivalent diameter (μm) of the particles and nis the number of the particles.

A sample is dissolved in a solvent which dissolves the polyester but notthe particles, the particles are separated from the resulting solutionby centrifugation, and the ratio (wt %) of the amount of the particlesto the total amount is taken as the content of the particles.

(2) Layer Thickness

Using a secondary ion mass spectrometer (SIMS), the concentration ratio(M⁺/C⁺) of an element derived from the highest concentration particlesout of the particles contained in the film of a portion from the surfacelayer up to a depth of 3,000 nm to the elemental carbon of the polyesteris taken as a concentration of particles, and the portion from thesurface layer up to a depth of 3,000 nm is analyzed in the thicknessdirection. The concentration of the particles is low in the surfacelayer which is an interface but becomes higher as the distance from thesurface increases. Once having reached the maximal value, theconcentration of the particles begins to decrease again. Based on thisconcentration distribution curve, a depth (deeper than a depth at whichthe concentration of the particles becomes maximal) at which theconcentration of the particles in the surface layer becomes half of themaximal value is taken as the thickness of the surface layer.

Measurement conditions are as follows.

(i) measurement instrument

secondary ion mass spectrometer (SIMS)

(ii) measurement conditions

species of primary ion: O₂ ⁺

primary ion acceleration voltage: 12 kV

primary ion current: 200 nA

luster area: 400 μm□

analytical area: 30% of gate

degree of vacuum for measurement: 0.8 Pa (6.0×10⁻³ Torr)

E-GUN: 0.5 kV-3.0 A

When the particles which are contained in the largest amount in an areafrom the surface layer to a depth of 3,000 nm are organic polymerparticles, it is difficult to measure them with SIMS. Therefore, thesame depth profile as described above may be measured by XPS (X-rayphotoelectron spectrometry), IR (infrared spectrometry) or the like,while etching from the surface, to obtain the thickness of the surfacelayer.

(3) Total Thickness of Film

Ten films are placed one upon another in such a manner that dust shouldnot be inserted therebetween, and the total thickness of the films ismeasured by an intermittent electronic micrometer to calculate thethickness of each film.

(4) Young's Modulus

The film is cut to a width of 10 mm and a length of 15 cm, and thissample is pulled by an Instron type universal tensile tester at a chuckinterval of 100 mm, a pulling rate of 10 mm/min and a chart rate of 500mm/min. The Young's modulus is calculated from the tangent of a risingportion of the obtained load-elongation curve.

(5) Electromagnetic Conversion Characteristics

The following commercially available devices are used to record a signalhaving a frequency of 7.4 MHz, the ratio of a 6.4 MHz value to a 7.4 MHzvalue of its reproduction signal is taken as the C/N of the tape, andthe relative values of Examples 1 to 7 and Comparative Examples 1 to 3are obtained when the C/N of Example 1 is ±0 dB and the relative valuesof Examples 8 to 13 and Comparative Examples 4 and 5 are obtained whenthe C/N of Example 10 is ±0 dB, and evaluated as follows.

⊚: +3 dB or more

∘: +1 to +3 dB

X: less than +1 dB

Used Devices

8 mm video recorder: EDV-6000 of Sony Corp.

C/N measurement: noise meter of Shibasoku Co., Ltd.

(6) Slit Yield

The film is slit to a width of 700 mm and a length of 7,000 m, and theslit yield when the film is wound round 20 or more rolls is obtained andevaluated based on the following criteria.

⊚: 90% or more

∘: 70% or more and less than 90%

X: less than 70%

(7) Surface Roughness (WRa)

Using the non-contact 3-D roughness meter (NT-2000) of WYKO Co., Ltd.,10 or more measurements (n) are made under such conditions as ameasurement area of 247 μm×188 μm (0.046 mm²) and a measurementmagnification of 25×, and the center plane surface roughness (WRa) iscalculated with surface analysis software incorporated in the roughnessmeter.

(A) Center Plane Average Roughness (WRa)

This is a value calculated from the following expression.${WRa} = {\sum\limits_{k = 1}^{M}\quad {\sum\limits_{j = 1}^{N}\quad {{{Z_{jk} - \overset{\_}{Z}}}/( {M \cdot N} )}}}$

provided$\overset{\_}{Z} = {\sum\limits_{k = 1}^{M}\quad {\sum\limits_{j = 1}^{N}\quad {Z_{jk}/( {M \cdot N} )}}}$

Z_(jk) is a height in the direction of the Z axis on the X and Y planesat a j-th position and a k-th position in the direction of the X axis(247 μm) and the direction of the Y axis (188 μm) perpendicular to thedirection when these directions are divided into M and N sections,respectively.

(8) Film Cost

This is judged based on the recovered polymer.

⊚: recovery (R) of 50% or more.

∘: recovery (R) of 30% or more and less than 50%.

Δ: recovery (R) of 10% or more and less than 30%.

X: recovery (R) of less than 10%.

(9) Friction Coefficient of Film

A glass plate is fixed under a set of two films, a lower film (film incontact with the glass plate) of the set is pulled with a low-speed roll(about 10 cm/min), and a detector is fixed at one end of an upper film(opposite end in the pulling direction of the lower film) to detectinitial tensile force between the films. A sled used has a weight of 1kg and a lower area of 100 cm².

The friction coefficient (μs) is obtained from the following equation:

μs=initial tensile force (kg)/load of 1 kg

(10) Measurement of Volume Resistivity

The volume resistivity of a molten film is measured using an apparatusshown in FIG. 2. A measurement sample 1 is a film having a thickness ofabout 150 μm. An upper electrode 3 having a diameter of 5.6 cm and athickness of 0.2 cm is placed above a cylindrical lower electrode 2having a diameter of 20 cm with a parallel space of 150 μm therebetween,and the measurement sample is inserted between these electrodes in sucha manner that it comes in close contact with these.

The lower electrode 2 incorporates a charger 4 and a temperaturedetection end 5 and differences in the surface temperature of the lowerelectrode on the measurement plane are controlled to 1° C. or less and adifference between the surface temperature of the lower electrode andthe temperature of the detection end portion is controlled to 2° C. orless at a temperature elevation rate of 8° C./min. The detectiontemperature is measured with a read thermometer 7. The whole electrodesare placed in a heat insulating container 11.

Voltage generated from a power source 8 is applied to the bothelectrodes through a standard resistor 9. When the DC volume resistivityof the film is to be measured, the power source generates DC 100 V andwhen the AC volume resistivity of the film is to be measured, the powersource generates 100 V at 50 Hz. A current running through the circuitis obtained by reading a voltage generated at both ends of the standardresistor with an electron meter 10 having an internal impedance of 100MΩ or more.

The AC volume resistivity of the film-like polymer is measured with theabove apparatus at a temperature elevation rate of the lower electrodeof 8° C./min by setting the temperature of the above electrode to themelting point of the polymer measured by DSC+30° C., and the AC volumeresistivity Z is obtained from the following equation based on appliedvoltage E, current I, electrode area S and space d between electrodes.$Z = {\frac{E}{I} \cdot \frac{S}{d}}$

Example 1

Dimethyl-2,6-naphthalate and ethylene glycol were polymerized inaccordance with a commonly used method by adding manganese acetate as anester exchange catalyst, antimony trioxide as a polymerization catalyst,phosphorous acid as a stabilizer and additive particles shown in Table 1as a lubricant to obtain a new chip for a flat layer (layer A) having anintrinsic viscosity (in o-chlorophenol, at 35° C.) of 0.61. Meanwhile, achip recovered from the laminate film itself and a new chip shown inTable 1 were used as chips for a rough layer (layer B) in a ratio shownin Table 1. A sulfonic acid quaternary phosphonium salt compound wascontained in the new chips for the layers A and B in an amount of 2 mmol%.

These polymers for the layers A and B were dried at 170° C. for 6 hours.The dried chips were supplied to the hoppers of two extruders in such aratio that the layer thickness structure shown in Table 1 was obtained,molten at a temperature of 280 to 300° C., laminated together with amulti-manifold coextrusion die in such a manner that the layer B wasplaced upon one side of the layer A, and extruded onto a rotary coolingdrum having a surface finish of about 0.3 s and a surface temperature of60° C. to obtain an unstretched laminate film having a thickness of 91μm.

This unstretched laminate film had an AC volume resistivity of 4×10⁸Ω·cm.

The thus obtained unstretched laminate film was preheated at 120° C.,stretched to 5.2 times between low-speed and high-speed rolls by heatingwith an IR heater having a surface temperature of 900° C. 15 mm fromabove, quenched and subsequently, supplied to a stenter to be stretchedto 3.9 times in a transverse direction at 145° C. The obtained biaxiallyoriented film was heat set with hot air heated at 210° C. for 4 secondsto obtain a biaxially oriented laminate polyester film having athickness of 4.5 μm. The film had a Young's modulus in a longitudinaldirection of 8,826 MPa (900 kg/mm²) and a Young's modulus in atransverse direction of 5,884 MPa (600 kg/mm²).

The following magnetic coating was applied to one side (layer A) of thisbiaxially oriented laminate polyester film to a thickness of 0.5 μm,subjected to alignment treatment in a DC magnetic field of 2,500 Gauss,dried by heating at 100° C., and to supercalendering (linear pressure of300 kg/cm, temperature of 80° C.) and wound up. The wound roll was keptin an oven heated at 55° C. for 3 days and cut to a width of 8 mm toobtain a magnetic tape.

Preparation of the Magnetic Coating

The following composition was placed in a ball mill to be kneaded for 16hours and dispersed, and 5 parts by weight of an isocyanate compound(Desmodur of Bayer AG) was added and dispersed by high-speed shearingfor 1 hour to obtain a magnetic coating.

composition of magnetic coating: parts by weight needle-like Feparticles 100 vinyl chloride-vinyl acetate copolymer 15 (Slec 7A ofSekisui Chemical Co., Ltd.) 5 thermoplastic polyurethane resin 5chromium oxide 5 carbon black 5 lecithin 2 fatty acid ester 1 toluene 50methyl ethyl ketone 50 cyclohexanone 50

The obtained magnetic tape was measured for its electromagneticconversion characteristics in accordance with the above measurementmethod. The results are shown in Table 1.

Comparative Examples 1 to 3

Biaxially oriented laminate polyester films were obtained in the samemanner as in Example 1 except that lubricant particles added, the layerthickness structure and the proportion of a recovered polymer werechanged as shown in Table 1. Magnetic tapes were obtained from theobtained biaxially oriented laminate polyester films in the same manneras in Example 1. The measurement results of characteristic properties ofthe films are shown in Table 1.

Examples 2 to 7

Laminate films were obtained in the same manner as in Example 1 exceptthat lubricant particles added, the layer thickness structure, theproportion of a recovered polymer and Young's moduli were changed asshown in Table 1. To obtain these Young's moduli, the draw ratio in alongitudinal direction was set to 5.1 times and the draw ratio in atransverse direction was set to 4.9 times in Examples 2, 4, 5 and 6, thedraw ratio in a longitudinal direction was set to 4.8 times and the drawratio in a transverse direction was set to 5.2 times in Example 3, andthe draw ratio in a longitudinal direction was set to 4.0 times and thedraw ratio in a transverse direction was set to 5.4 times in Example 7.

Magnetic tapes were obtained from the obtained biaxially orientedlaminate polyester films in the same manner as in Example 1. Themeasurement results of characteristic properties are shown in Table 1.

As is obvious from Table 1, the biaxially oriented laminate polyesterfilms of the present invention have excellent characteristic propertiessuch as electromagnetic conversion characteristics, winding propertiesand film cost, as a base film for a high-density magnetic recordingmedium.

TABLE 1 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 layer thickness structure totalthickness μm 4.5 6.0 4.5 6.0 6.0 thickness of layer A μm 1.5 2.0 1.5 2.02.0 thickness of layer B μm 3.0 4.0 3.0 4.0 4.0 proportion of thicknessof layer B % 67 67 67 67 67 Young's moduli longitudinal direction MPa8826 7846 6865 7846 7846 transverse direction MPa 5884 6375 7159 63756375 self-recycling method layer from which recovered polymer is layer Blayer B layer B layer B layer B obtained layer A proportion of polymerof layer A % 33 33 33 33 33 composition of polymer of layer A particlesII species of lubricant spherical spherical spherical sphericalspherical silica silica silica silica silica average particle diameterμm 0.14 0.14 0.14 0.14 0.14 amount added wt % 0.10 0.10 0.15 0.10 0.10layer B proportion of polymer of layer B % 67 67 67 67 67 proportion ofrecovered polymer % 50 50 50 50 50 proportion of new polymer % 17 17 1717 17 composition of polymer of layer B composition of new polymerparticle I species of lubricant spherical spherical spherical sphericalspherical silica silica silica silica silica average particle diameterμm 0.30 0.30 0.30 0.30 0.30 amount added wt % 0.300 0.300 0.400 0.1000.500 particle II species of lubricant spherical spherical sphericalspherical spherical silica silica silica silica silica average particlediameter μm 0.14 0.14 0.14 0.14 0.14 amount added wt % 0.300 0.300 0.3500.300 0.300 final composition of polymer of layer B amount of particlesI wt % 0.150 0.150 0.200 0.050 0.250 amount of particles II wt % 0.2000.200 0.250 0.200 0.200 average particle diameter of all particles μm0.15 0.15 0.15 0.14 0.16 t_(B/)d_(B) — 20 27 20 29 25 surface roughnesslayer A WRa(A) nm 6.1 5.7 5.0 4.1 7.6 layer B WRa(B) nm 11.2 11.5 11.56.9 16.8 film friction coefficient — 0.35 0.34 0.34 0.38 0.31electromagnetic conversion characteristics — ±0 db ∘ ∘ ⊚ Δ slit yield —∘ ∘ ∘ Δ ⊚ film cost — ⊚ ⊚ ⊚ ⊚ ⊚ Ex. 6 Ex. 7 C. Ex. 1 C. Ex. 2 C. Ex. 3layer thickness structure total thickness μm 6.0 4.5 4.5 4.5 6.0thickness of layer A μm 1.0 2.0 3.5 0.3 2.0 thickness of layer B μm 5.02.5 1.0 4.2 4.0 proportion of thickness of layer B % 83 56 22 93 67Young's moduli longitudinal direction MPa 7846 5394 8826 8826 8826transverse direction MPa 6375 11768 5884 5884 5884 self-recycling methodlayer from which recovered polymer is layer B layer B layer B layer Blayer B obtained layer A proportion of polymer of layer A % 17 44 78 733 composition of polymer of layer A particles II species of lubricantspherical spherical spherical spherical spherical silica silica silicasilica silica average particle diameter μm 0.09 0.20 0.14 0.14 0.06amount added wt % 0.15 0.05 0.10 0.10 0.10 layer B proportion of polymerof layer B % 83 56 22 93 67 proportion of recovered polymer % 70 40 5 8050 proportion of new polymer % 13 16 17 13 17 composition of polymer oflayer B composition of new polymer particle I species of lubricantspherical crosslinked spherical spherical spherical silica siliconesilica silica silica average particle diameter μm 0.25 0.50 0.30 0.300.30 amount added wt % 0.300 0.060 0.184 0.210 0.300 particle II speciesof lubricant spherical spherical spherical spherical spherical silicasilica silica silica silica average particle diameter μm 0.09 0.20 0.140.14 0.06 amount added wt % 0.450 0.350 0.223 0.240 0.300 finalcomposition of polymer of layer B amount of particles I wt % 0.160 0.0280.150 0.150 0.150 amount of particles II wt % 0.310 0.190 0.200 0.2000.200 average particle diameter of all particles μm 0.09 0.20 0.15 0.150.06 t_(B/)d_(B) — 58 13 7 28 67 surface roughness layer A WRa(A) nm 7.95.0 3.4 10.2 2.7 layer B WRa(B) nm 12.4 7.5 9.3 11.8 9.5 film frictioncoefficient — 0.40 0.41 0.36 0.34 0.46 electromagnetic conversioncharacteristics — Δ ∘ ⊚ x ⊚ slit yield — Δ Δ ∘ ∘ x film cost — ⊚ ∘ x ⊚ ⊚Ex.: Example C. Ex.: Comparative Example

Example 8

Dimethyl-2,6-naphthalate and ethylene glycol were polymerized inaccordance with a commonly used method by adding manganese acetate as anester exchange catalyst, antimony trioxide as a polymerization catalyst,phosphorous acid as a stabilizer and additive particles shown in Table 2as a lubricant to obtain a new chip for a flat layer (layer A) having anintrinsic viscosity (in o-chlorophenol, at 35° C.) of 0.61. Meanwhile, achip recovered from the laminate film itself and a new chip shown inTable 2 were used as chips for a rough layer (layer B) in a ratio shownin Table 2. A sulfonic acid quaternary phosphonium salt compound wascontained in the new chips for the layers A and B in an amount of 2 mmol%.

These polymers for the layers A and B were dried at 170° C. for 6 hours.The dried chips were supplied to the hoppers of two extruders in such aratio that the layer thickness structure shown in Table 2 was obtained,molten at a temperature of 280 to 300° C., laminated together with amulti-manifold coextrusion die in such a manner that the layer B wasplaced upon one side of the layer A, and extruded onto a rotary coolingdrum having a surface finish of about 0.3 s and a surface temperature of60° C. to obtain an unstretched laminate film having a thickness of 212μm.

This unstretched laminate film had an AC volume resistivity of 4×10⁸Ω·cm.

The thus obtained unstretched laminate film was preheated at 120° C.,stretched to 5.1 times between low-speed and high-speed rolls by heatingwith an IR heater having a surface temperature of 900° C. 15 mm fromabove, quenched and supplied to a stenter to be stretched to 4.9 timesin a transverse direction at 145° C. The obtained biaxially orientedfilm was heat set with hot air heated at 210° C. for 4 seconds to obtaina biaxially oriented laminate polyester film having a thickness of 4.5μm. The film had a Young's modulus in a longitudinal direction of 7,846MPa (800 kg/mm²) and a Young's modulus in a transverse direction of6,375 MPa (650 kg/mm²).

The following magnetic coating was applied to one side (layer A) of thisbiaxially oriented laminate polyester film to a thickness of 0.2 μm,subjected to alignment treatment in a DC magnetic field of 2,500 Gauss,dried by heating at 100° C., and to supercalendering (linear pressure of300 kg/cm, temperature of 80° C.) and wound up. The wound roll was keptin an oven heated at 55° C. for 3 days and cut to a width of 8 mm toobtain a magnetic tape.

The obtained magnetic tape was measured for its electromagneticconversion characteristics in accordance with the above measurementmethod. The results are shown in Table 2.

Examples 9 to 15 and Comparative Examples 4 and 5

Laminate films were obtained in the same manner as in Example 8 exceptthat lubricant particles added, the layer thickness structure, theproportion of a recovered polymer and Young's moduli were changed asshown in Table 2. To obtain these Young's moduli, the draw ratio in alongitudinal direction was set to 5.1 times and the draw ratio in atransverse direction was set to 4.9 times in Example 9, the draw ratioin a longitudinal direction was set to 5.2 times and the draw ratio in atransverse direction was set to 3.9 times in Examples 11 to 13 andComparative Examples 4 and 5, the draw ratio in a longitudinal directionwas set to 4.8 times and the draw ratio in a transverse direction as setto 5.2 times in Example 10, the draw ratio in a longitudinal directionwas set to 3.5 times and the draw ratio in a transverse direction as setto 5.8 times in Example 14, and the draw ratio in a longitudinaldirection was set to 4.0 times and the draw ratio in a transversedirection as set to 5.4 times in Example 15.

Magnetic tapes were obtained from the obtained biaxially orientedlaminate polyester films in the same manner as in Example 8. Themeasurement results of characteristic properties are shown in Table 2.

As is obvious from Table 2, the biaxially oriented laminate polyesterfilms of the present invention have excellent characteristic propertiessuch as electromagnetic conversion characteristics, winding propertiesand film cost, as a base film for a high-density magnetic recordingmedium.

TABLE 2 Ex. 8 Ex. 9 Ex. 10 Ex. 11 Ex. 12 layer thickness structure totalthickness μm 8.5 6.0 6.0 6.0 8.5 thickness of layer A μm 6.7 4.2 4.2 3.24.5 thickness of layer B μm 1.8 1.8 1.8 2.8 4.0 proportion of thicknessof layer B % 21 30 30 47 47 Young's moduli longitudinal direction MPa7846 7846 6865 8826 8826 transverse direction MPa 6375 6375 7159 58845884 self-recycling method layer from which recovered polymer is layer Blayer B layer B layer B layer B obtained layer A proportion of polymerof layer A % 79 70 70 53 53 composition of polymer of layer A particlesII species of lubricant spherical spherical spherical sphericalspherical silica silica silica silica silica average particle diameterμm 0.14 0.14 0.14 0.14 0.09 amount added wt % 0.01 0.01 0.10 0.10 0.15layer B proportion of polymer of layer B % 21 30 30 47 47 proportion ofrecovered polymer % 10 15 15 35 35 proportion of new polymer % 11 15 1512 12 composition of polymer of layer B composition of new polymerparticle I species of lubricant spherical spherical spherical sphericalspherical silica silica silica silica silica average particle diameterμm 0.30 0.30 0.30 0.30 0.25 amount added wt % 0.425 0.255 0.255 0.3900.406 particle II species of lubricant spherical spherical sphericalspherical sphetical silica silica silica silica silica average particlediameter μm 0.14 0.14 0.14 0.14 0.09 amount added wt % 0.385 0.333 0.2700.360 0.556 final composition of polymer of layer B amount of particlesI wt % 0.250 0.150 0.150 0.150 0.160 amount of particles II wt % 0.2300.200 0.200 0.200 0.310 average particle diameter of all particles μm0.16 0.15 0.15 0.15 0.10 t_(B)/d_(B) — 11 12 12 19 40 surface roughnesslayer A WRa(A) nm 2.2 1.5 3.2 4.4 4.3 layer B WRa(B) nm 10.5 10.7 10.610.9 11.7 film friction coefficient — 0.40 0.43 0.35 0.34 0.38electromagnetic conversion characteristics — ⊚ ⊚ ±0 db Δ Δ slit yield —Δ Δ ∘ ∘ Δ film cost — Δ Δ Δ ∘ ∘ Ex. 13 Ex. 14 Ex. 15 C. Ex. 4 C. Ex. 5layer thickness structure total thickness μm 6.0 6.0 4.5 6.0 6.0thickness of layer A μm 4.2 4.2 2.7 5.3 3.2 thickness of layer B μm 1.81.8 1.8 0.7 2.8 proportion of thickness of layer B % 30 30 40 12 47Young's moduli longitudinal direction MPa 8826 5394 5884 8826 8826transverse direction MPa 5884 11768 8826 5884 5884 self-recycling methodlayer from which recovered polymer is layer B layer B layer B layer Blayer B obtained layer A proportion of polymer of layer A % 70 70 60 8853 composition of polymer of layer A particles II species of lubricantspherical spherical spherical spherical spherical silica silica silicasilica silica average particle diameter μm 0.14 0.14 0.14 0.14 0.06amount added wt % 0.10 0.10 0.01 0.10 0.10 layer B proportion of polymerof layer B % 30 30 40 12 47 proportion of recovered polymer % 15 15 25 535 proportion of new polymer % 15 15 15 7 12 composition of polymer oflayer B composition of new polymer particle I species of lubricantspherical crosslinked spherical spherical spherical silica siliconesilica silica silica average particle diameter μm 0.30 0.50 0.30 0.300.30 amount added wt % 0.425 0.051 0.500 0.249 0.390 particle II speciesof lubricant spherical spherical spherical spherical spherical silicasilica silica silica silica average particle diameter μm 0.14 0.14 0.140.14 0.06 amount added wt % 0.270 0.270 0.450 0.266 0.360 finalcomposition of polymer of layer B amount of particles I wt % 0.250 0.0300.250 0.150 0.150 amount of particles II wt % 0.200 0.200 0.230 0.2000.200 average particle diameter of all particles μm 0.16 0.14 0.16 0.150.06 t_(B)/d_(B) — 11 13 11 5 47 surface roughness layer A WRa(A) nm 4.22.3 2.2 2.5 2.1 layer B WRa(B) nm 15.9 6.3 10.7 8.7 9.2 film frictioncoefficient — 0.32 0.39 0.42 0.35 0.46 electromagnetic conversioncharacteristics — Δ ⊚ ⊚ ⊚ ⊚ slit yield — ⊚ Δ Δ ∘ x film cost — Δ ∘ Δ x ∘Ex.: Example C. Ex.: Comparative Example

What is claimed is:
 1. A biaxially oriented laminate polyester filmcomprising a first polyester layer and a second polyester layer, whereinthe first polyester layer has a thickness (t_(A)) of 0.3 to 5 μm, thesecond polyester layer contains an inert fine particle lubricant and hasa thickness (t_(B)) of 1.5 to 9 μm, and the first polyester layer andthe second polyester layer satisfy the following expressions (1) to (4):WRa(B)>WRa(A)  (1) 0.5≦t _(B) /t≦0.9  (2) 10<t _(B) /d _(B)≦60  (3) t=3to 10 μm  (4) wherein WRa(A) is the center plane average roughness (nm)of the exposed surface of the first polyester layer, WRa(B) is thecenter plane average roughness (nm) of the exposed surface of the secondpolyester layer, t_(B) is the thickness (μm) of the second polyesterlayer, t is the sum of t_(A) and t_(B), t_(A) is the thickness (μm) ofthe first polyester layer, and d_(B) is the average particle diameter(μm) of the inert fine particle lubricant contained in the secondpolyester layer.
 2. The biaxially oriented laminate polyester film ofclaim 1, wherein WRa(A) is in the range of 3 to 8 nm and WRa(B) is inthe range of 6 to 18 nm.
 3. The biaxially oriented laminate polyesterfilm of claim 1, wherein the intrinsic viscosity of the polyester of thesecond polyester layer is lower than the intrinsic viscosity of thepolyester of the first polyester layer.
 4. The biaxially orientedlaminate polyester film of claim 1, wherein the polyester of the secondpolyester layer comprises a recovered polyester having the samecomposition as a recovered laminate polyester film which is thebiaxially oriented laminate polyester film of claim 1 or an unstretchedfilm thereof.
 5. The biaxially oriented laminate polyester film of claim4, wherein the thickness ratio of the first polyester layer to thesecond polyester layer is the same as the thickness ratio of the firstpolyester layer to the second polyester layer of the recovered laminatepolyester film.
 6. The biaxially oriented laminate polyester film ofclaim 1, wherein the second polyester layer is formed from a recoveredlaminate polyester film and a new polyester to ensure that theconcentration (C_(Bi), wt %) of the inert fine particle lubricantcontained in the second polyester layer should satisfy the followingequation: C _(Bi)=(C _(Ai) ×t _(A) ×R+100×C _(vi)×(t _(B)−(t _(A) +t_(B))×R/100))/(t _(B)×(100−R))) wherein C_(Ai) is the concentration (wt%) of an inert fine particle lubricant contained in the first polyesterlayer of the recovered laminate polyester film, C_(vi) is theconcentration (wt %) of an inert fine particle lubricant contained inthe new polyester used together with the recovered laminate polyesterfilm for the formation of the second polyester layer, t_(A) is thethickness (μm) of the first polyester layer of the recovered laminatepolyester film, t_(B) is the thickness (μm) of the second polyesterlayer of the recovered laminate polyester film, and R is the proportion(wt %) of the recovered laminate polyester film used together with thenew polyester for the formation of the second polyester layer.
 7. Thebiaxially oriented laminate polyester film of claim 6, wherein R is 1 to90 wt %.
 8. The biaxially oriented laminate polyester film of claim 6which has the same laminate structure and composition as the recoveredlaminate polyester film.
 9. The biaxially oriented laminate polyesterfilm of claim 1, wherein the second polyester layer contains two or morekinds of inert fine particle lubricants having different averageparticle diameters which (i) are of two or more different chemicalspecies or (ii) are of the same chemical species and havedistinguishable particle size distributions.
 10. The biaxially orientedlaminate polyester film of claim 1, wherein the Young's moduli inlongitudinal and transverse directions of the film are both in the rangeof 4,413 to 19,614 MPa (450 to 2,000 kg/mm²) and the ratio of theYoung's modulus in a longitudinal direction to the Young's modulus in atransverse direction is in the range of 0.3 to 2.5.
 11. The biaxiallyoriented laminate polyester film of claim 1, wherein a polyester formingthe first polyester layer and the second polyester layer ispolyethylene-2,6-naphthalene dicarboxylate.
 12. The biaxially orientedlaminate polyester film of claim 1, wherein a polyester forming thefirst polyester layer and/or the second polyester layer contains, as thecopolymerized component, a sulfonic acid quaternary phosphonium salt inan amount of 0.02 to 45 mmol % and has an AC volume resistivity of 1×10⁶to 9×10⁸ Ω·cm.
 13. The biaxially oriented laminate polyester film ofclaim 1 which is a base film for a magnetic recording tape for digitalrecording.
 14. The biaxially oriented laminate polyester film of claim 1which is a base film for a magnetic recording tape for data storage. 15.A method of producing the biaxially oriented laminate polyester film ofclaim 1 by biaxially stretching an unstretched laminate polyester filmconsisting of a first unstretched polyester layer and a secondunstretched polyester layer, wherein a recovered laminate polyester filmand a new polyester are used to form the second unstretched polyesterlayer under the condition that the following expression should besatisfied: C _(Bi)=(C _(Ai) ×t _(A) ×R+100×C _(vi)(t _(B)−(t_(A) +t_(B))×R/100))/(t _(B)×(100−R)) wherein C_(Bi) is the concentration (wt%) of an inert fine particle lubricant contained in the secondunstretched polyester layer, C_(Ai) is the concentration (wt %) of aninert fine particle lubricant contained in the first polyester layer ofthe recovered laminate polyester film, C_(vi) is the concentration (wt%) of an inert fine particle lubricant contained in the new polyesterused together with the recovered laminate polyester film for theformation of the second polyester layer, t_(A) is the thickness (μm) ofthe first polyester layer of the recovered laminate polyester film,t_(B) is the thickness (μm) of the second polyester layer of therecovered laminate polyester film, and R is the proportion (wt %) of therecovered laminate polyester film used together with the new polyesterfor the formation of the second polyester layer.